JP5048872B2 - Projection exposure method, projection exposure apparatus, laser radiation source, and bandwidth narrowing module for laser radiation source - Google Patents

Description

  The present invention relates to a projection exposure method for exposing a radiation-sensitive substrate arranged in an image plane area of a projection objective by means of at least one image of a mask pattern arranged in an object plane area of the projection objective, The invention relates to a projection exposure apparatus suitable for carrying out the method, and also to a laser radiation source and a bandwidth narrowing module for the laser radiation source.

  Today, microlithographic projection exposure methods are mainly used to manufacture semiconductor components and other microstructured devices. In this case, a mask (reticle) is used which carries the pattern of the structure to be imaged, for example the line pattern of the layer of the semiconductor component. The mask is positioned in the region of the object plane of the projection objective between the illumination system and the projection objective in the projection exposure apparatus and illuminated with illumination radiation supplied by the illumination system. Radiation modified by the mask and pattern passes through the projection objective as projection radiation, which images the mask pattern onto the exposed substrate, usually carrying a radiation-sensitive layer (photoresist). To do.

Current projection exposure apparatus for high resolution microlithography in the deep ultraviolet or ultra deep ultraviolet (DUV or VUV) range generally uses a laser as the primary light source. In particular, a KrF excimer laser having an operating wavelength of about 248 nm or an ArF excimer laser having an operating wavelength of about 193 nm is conventional. An F ₂ laser can be used at an operating wavelength of 157 nm, and an Ar ₂ excimer laser can be used at 126 nm. The primary laser radiation source emits a laser beam comprised of laser light, which is received by a connected illumination system and reshaped to generate illumination radiation that is directed onto the mask. The spectral characteristics of the illumination radiation and the projection radiation, i.e. the characteristics of these radiation depending on the wavelength λ or the angular frequency ω, are in this case substantially determined by the spectral characteristics of the primary laser radiation.

  In the design of a lithography process, the linewidth of the structure on the reticle is imaged using illumination that is assumed to be known using a projection objective so that the desired structure size is exposed in the photosensitive layer. Adjusted. In this case, it is important that an equal structure of the mask is imaged equally in the photoresist regardless of its position on the substrate. Otherwise, in the case of semiconductor components, there may be a speed loss or even a loss of function in the worst case. Thus, one important variable in semiconductor manufacturing is the change in critical structure thickness (CD), introduced by the process, also referred to as “critical dimension change” or “CD change”. From this, a uniform width across the field of equal structure being imaged, the so-called CD uniformity, constitutes a very important quality criterion of the lithography process.

  The decision factor for the width of the structure in the photoresist is the radiation energy deposited there. By convention, it is assumed that the photoresist is exposed above a certain amount of radiation energy deposition and not below this amount. The limit value for the amount of radiation energy is also referred to as a “resist threshold”. In this case, what is important is the radiation intensity integrated over a total exposure time at a location on the substrate. The magnitude of the radiation energy deposited at a particular location in the photoresist depends on a number of influence variables, in particular optical aberrations, in particular chromatic aberration, the polarization state of the exposure radiation, and also the effects of stray light and double reflection. If a laser is used as the primary light source, so-called speckle, which may result in self-interference when using at least partially coherent radiation, is added as yet another potential cause of CD variation. there is a possibility. In this case, mainly so-called time-varying speckles (dynamic speckles, time speckles) are important, and these speckles use a light source with a coherence time not much shorter than the duration of the laser pulse. Caused by temporal intensity fluctuations resulting from In contrast, a typical non-coherent light source has a very short coherence time, in which case time-varying speckle is not a problem.

  A number of influence parameters should be taken into account when selecting a suitable laser radiation source for a microlithographic projection exposure apparatus. Color correction is very complex, especially for high resolution projection objectives with large image fields. If the projection objective is not fully color corrected, radiation having different wavelengths will result in different focal positions for each wavelength within the image field of the projection objective. In order to avoid the disadvantages obtained, it is common practice to use a very narrow band laser radiation source. Therefore, a conventional excimer laser includes a bandwidth narrowing module that narrows the spontaneous emission spectrum of a laser of several hundred pm to a bandwidth narrower than 1 pm, for example, by a power of ten. Thus, a large bandwidth is disadvantageous with respect to chromatic aberration. In contrast, a wide bandwidth is rather preferred for the generation of dynamic speckle, since an undesirable interference phenomenon called speckle is thought to be due to the temporal coherence of light. Since this light is not very coherent, different wavelengths are included in the laser radiation and a wide bandwidth is preferred with respect to avoiding speckle.

  Thus, the selection of an appropriate bandwidth is always a compromise between the requirement for low chromatic aberration on the one hand and a few speckles on the other hand. Optimum bandwidth setting is a technical problem that should be determined when designing each projection exposure apparatus, for example, based on data available for color correction of the projection objective.

  The patent applications US 2006/0146310 A1 and US 2008/0225921 A1 disclose in each case a projection exposure method and a projection exposure apparatus which can set or change the spectral intensity distribution of the laser radiation source used.

US 2006/0146310 A1 US 2008/0225921 A1

  It is an object of the present invention to operate using a laser as the primary light source and reduce the effect of time-varying speckle on image generation as compared to conventional systems and methods, while not increasing the effect of chromatic aberration on image generation. It is to provide a designed projection exposure method and a projection exposure apparatus designed to carry out the method. Yet another object is to provide a laser radiation source suitable for implementing the above-described projection exposure method, and also a bandwidth narrowing module for the laser radiation source.

  The above object uses a projection exposure method comprising the features of claim 1, uses a projection exposure apparatus comprising the features of claim 6, and includes a laser radiation source comprising the features of claim 11 and the features of claim 14. This is achieved using a bandwidth narrowing module. Advantageous developments are specified in the dependent claims. The language of all claims is incorporated by reference into the content of this description.

  By appropriate setting of the spectral shape of the laser line, ie by setting the spectral intensity distribution of the laser radiation emitted by the primary laser radiation source, the influence of the laser radiation source on chromatic aberration and the laser radiation on the generation of dynamic speckles. It is recognized that the balance between source effects can be influenced in a targeted manner.

  Setting the appropriate spectral shape of the laser line constitutes a degree of freedom not previously considered in the design of lithographic apparatus and processes.

  The spectral intensity distribution of the laser lines used has two opposite effects on the lithography process, wide laser lines increase chromatic aberration, whereas narrow laser lines result in long coherence times. Therefore, it can lead to severe dynamic speckle. So far, essentially only chromatic aberration has been the driving force in laser development projects. Therefore, considerable efforts have been made to develop laser radiation sources with the narrowest bandwidth possible. To come here and get the original true reading with an acceptable small CD change, the dynamic speckle must be reduced considerably as well, thus reducing both effects It is recognized that this should be taken into account.

  The formulation method of the present invention described above explains how dynamic speckle can be minimized by adjusting the spectral shape of the laser line without degrading chromatic aberration in the process. This formula can be understood as follows.

に基づくと、この場合に対応する全ての関連においてｄλとｄωとは比例し（ｄａ｜ｄω｜≪ω）、従って、角周波数と波長とは完全に均等な表記である。更に、以下では、レーザ線は、角周波数ω＝０にその中心を有すると仮定し、これは、そうしないと以下に示す式（１）以降においてその度にω²の代わりに（ω−ω₀）²と表記しなければならなくなるからである。 Let I (ω) be the spectral shape of the laser line. For reasons of more concise notation, the angular frequency ω is used in the following instead of the wavelength λ. Let c _{0 be} the speed of light,

Based on, dλ and dω are proportional (da | dω | << ω) in all relations corresponding to this case, and therefore the angular frequency and the wavelength are completely equivalent. Furthermore, in the following, it is assumed that the laser line has its center at the angular frequency ω = 0, which is otherwise (ω−ω instead of ω ² each time in the following equation (1) and thereafter. ₀ ) because it must be expressed as ² .

Chromatic aberration (or the effect of chromatic aberration in the lithography process) is proportional to the second moment of the spectral distribution, that is, the following equation holds.

In contrast, dynamic speckle is determined by the time autocorrelation μ (Δt). The coherence time τ is given by

When the coherence time τ is small, the dynamic speckle is weak, because in this case, averaging can be performed over a more static independent radiation distribution within the laser pulse. The autocorrelation can be calculated from the normalized spectral intensity distribution using a Fourier transform (FT).

As is well known, the Fourier transform maintains the L ₂ norm, so equation (2) is equal to:

Therefore, in the lithography process, the formula (1) and the formula (4) are optimal when the spectral line shape I (ω) as small as possible is used. From a unit perspective, it is clear that the dimensionless product ατ ² is the corresponding variable, ie, the smaller the product ατ ² , the more suitable the line shape is for the lithography process. Since this product parameterizes the spectral shape of the laser line, in the following this product will also be referred to as “line shape parameter” ατ ² or simply “shape parameter” ατ ² .

In some embodiments, the condition ατ ² <0.1 holds for the line shape parameter ατ ² . In particular, the condition ατ ² ≦ 0.09 or ατ ² ≦ 0.08 can be satisfied.

  For a better understanding of the background, some principles regarding spectral line width and profile are first described below. For the emission of electromagnetic radiation based on a transition between two energy levels of an atomic system, the frequency of the corresponding spectral line is not strictly a single wavelength. An intensity frequency distribution of the emission intensity is observed before and after the center frequency. The frequency interval at which the intensity decreases to half the maximum intensity existing at the center frequency between these two frequencies is called the full width at half maximum (FWHM). In this application, the term “bandwidth” is also used for the full width at half maximum. Generally, the spectral range within the full width at half maximum is called a line core, and the regions on both sides outside the line core are called line wings.

  The excited atoms' electrons can emit their excitation energy again in the form of electromagnetic radiation (spontaneous emission). During spontaneous emission, atoms do not emit strictly single wavelength radiation, and the emitted radiation has an internal distribution that is frequency dependent. In the absence of external influences, the atoms emit radiation having a so-called “natural line shape” represented by a Lorentz profile. The full width at half maximum of the Lorentz profile is the natural line width of the release process.

  However, in the field of laser physics, the Lorentz profile is only a rough approximation that is valid in the immediate vicinity of the center of the line, ie around the center frequency. A practical approach widely used in laser physics is that of a modified Lorentz curve that can serve as a practical reference in comparing different laser line shapes. This is described in more detail below in the description of the preferred exemplary embodiment.

A significant reduction of the line shape parameter ατ ² compared to the line shape parameter for the modified Lorentz curve can be obtained when the spectral intensity distribution I (ω) substantially corresponds to a Gaussian curve of width σ. , in the case of a Gaussian curve, α = σ ^2/2 and τ = 1 / (√2π σ)
Holds. In particular, a small deviation or error in fitting a Gaussian curve to the measured intensity profile, such as less than 10%, less than 8%, less than 5%, or less than 2%. The spectral intensity distribution I (ω) substantially corresponds to a Gaussian curve.

If the spectral intensity distribution I (ω) has a substantially parabolic shape, the line shape parameter ατ ² can be further reduced. In particular, in fitting the parabola to the measured intensity profile, small deviations or errors, eg, less than 15%, less than 10%, less than 8%, less than 5%, or less than 2% If only a small error is produced, the spectral intensity distribution I (ω) substantially corresponds to the parabolic shape. Compared to a Gaussian curve or a modified Lorentz curve, in the case of a parabolic shape, there is likewise only a low radiation energy in the region of the line wing, and accordingly a high radiation energy in the region of the line core, This proves that it is suitable for optimization concerning chromatic aberration and speckle. In particular, the side edges of the parabola intensity profile do not have any inflection points in the profile I (ω), so that in the case of a parabola distribution there is a relatively large amount of energy in the line core region and within the region of the line wing It is very clear that there is relatively little energy in

The invention also relates to a projection exposure apparatus including a primary laser radiation source for emitting laser radiation for which the condition ατ ² ≦ 0.3.

The invention also relates to a laser radiation source for emitting laser radiation for which the condition ατ ² ≦ 0.3.

The present invention is a bandwidth narrowing module for a laser radiation source, such that a laser radiation source equipped with this bandwidth narrowing module emits laser radiation for which the condition ατ ² ≦ 0.3. It also relates to the designed bandwidth narrowing module.

  The features described above and further features will become apparent not only from the claims but also from the description and the drawings, and the individual features may in each case be these features themselves or the embodiments and others of the invention. Embodiments can be achieved that can be achieved in the form of a plurality of subcombinations in the field, and that can be advantageously and essentially protected. Exemplary embodiments of the present invention are illustrated in the drawings and are described in more detail below.

It is the schematic of the structure of the projection exposure apparatus for microlithography. 1 is a schematic diagram of a spectral line profile of a laser. FIG. FIG. 6 is a diagram of the effect of the finite spectral bandwidth of a laser radiation source on the focal position when using a projection objective that is not fully color corrected. It is a graph regarding the spectrum characteristic of the laser radiation of the conventional laser. FIG. 4 is a schematic diagram of different possible spectral line shapes of laser radiation. It is the schematic of the structure of the excimer laser which has a bandwidth narrowing module.

FIG. 1 can be used for the manufacture of semiconductor components and other microstructured devices, and light or electromagnetic radiation from the deep ultraviolet range (VUV) to obtain fine resolutions down to a fraction of a micrometer. 1 shows an example 100 of a microlithographic projection exposure apparatus that operates using An ArF excimer laser having an operating wavelength of about 193 nm functions as the primary light source 102, and the linearly polarized laser beam of this laser is taken into the illumination system 190 coaxially with the optical axis 103 of the illumination system. Other UV laser radiation sources are also possible, for example an F ₂ laser having an operating wavelength of 157 nm, or a KrF excimer laser having an operating wavelength of 248 nm.

  The polarized light from the light source 102 is initially incident into a beam expander 104 that functions, for example, to reduce coherence and expand the beam cross section. The expanded laser beam is incident into a pupil shaping unit 150, which includes a plurality of optical components and optical groups, and a defined local (sometimes referred to as a secondary light source or “illumination pupil”). It is designed to generate a (two-dimensional) illumination intensity distribution in the pupil shaping surface 110 of the illumination system 190 downstream. The pupil shaping surface 110 is a pupil surface of the illumination system.

  The pupil shaping unit 150 can be set in a variable manner so that different local illumination intensity distributions (ie different structured secondary light sources) can be set depending on the drive of the pupil shaping unit. FIG. 1 schematically shows various illuminations of a circular illumination pupil, namely a conventional setting CON, a dipole illumination DIP or a quadrupole illumination QUAD with a centrally located circular illumination spot.

  An optical raster element 109 is disposed in the immediate vicinity of the pupil shaping surface 110. A combined optical unit 125 disposed downstream of the optical raster element 109 transmits light onto the intermediate image plane 121 and within the intermediate image plane 121 acts as a reticle / mask system (REMA) 122 that functions as an adjustable field stop. Is placed. The optical raster element 109, also referred to as the field definition element FDE, has a two-dimensional array of diffractive or refractive optical elements, and the incident radiation is rectangular within the region of the field plane 121 after passing through the downstream coupling optical unit 125. The incident radiation is shaped to shape the illumination field. Furthermore, the radiation is homogenized by the superposition of the partial beam bundles, so the FED functions as a field shaping and homogenizing element.

  The downstream imaging objective 140 (also called REMA objective) has an intermediate field plane 121 with a field stop 122 on the reticle 160 (mask, lithography original), for example between 2: 1 and 1: 5. There may be, and in this embodiment the image is on a scale that is approximately 1: 1.

  The optical components that receive light from the laser 102 and shape the illumination radiation that is provided on the reticle 160 from this light belong to the illumination system 190 of the projection exposure apparatus.

  Downstream of the illumination system, a pattern arranged on the reticle is located in the object plane 165 of the projection objective 170, and this pattern is scanned in this plane by using a scanning driver in the plane of the optical axis 103 (z direction). A device 171 for carrying and manipulating the reticle 160 is arranged so that it can be moved in a scanning direction (y direction) perpendicular to.

  Downstream of the reticle plane 165 is a wafer that functions as a reduction objective and the image of the pattern placed on the mask 160 is coated with a photoresist layer on a reduced scale, eg, 1: 4 or 1: 5 scale. A projection objective 170 imaging on 180 follows, and the photosensitive surface of this wafer is located in the image plane 175 of the projection objective 170. Refraction, catadioptric or reflective projection objectives are possible. Other reduction scales are possible, for example high reductions up to 1:20 or 1: 200.

  In this example, the substrate to be exposed, which is a semiconductor wafer 180, is carried by a device 181 that includes a scanner driver for moving the wafer in synchronism with the reticle 160 and perpendicular to the optical axis. Based on the design of the projection objective 170 (eg, refraction, catadioptric or reflection, no intermediate image or intermediate image, no fold or no fold), these movements should be performed in a parallel or anti-parallel manner to each other. Can do. A device 181, also called a “wafer stage”, and a device 171, also called a “reticle stage”, are part of a scanner device that is controlled using a scanning control device.

  The pupil shaping surface 110 is located at or near the optically conjugate position with respect to the nearest downstream pupil surface 145 and the image side pupil surface 172 of the projection objective 170. As a result, the spatial (local) light distribution in the pupil 172 of the projection objective is determined by the spatial light distribution (spatial distribution) in the pupil shaping surface 110 of the illumination system. Between the pupil planes 110, 145, and 172, a field plane that is a Fourier transform plane for each pupil plane is positioned in the optical beam path. In particular, the defined spatial distribution of illumination intensity in the pupil shaping surface 110 produces a specific angular distribution of illumination radiation in the region of the downstream field plane 121, and this angular distribution is further on the reticle 160. It means to correspond to a specific angular distribution of incident illumination radiation.

  The laser radiation source 102 emits laser radiation having a specific spectral intensity profile I (ω) that depends on the wavelength λ or the angular frequency ω. The effect of using a light source with a finite bandwidth on the image quality when using a projection objective that is not fully color corrected is schematically described with reference to FIG.

For this purpose, FIG. 2A schematically shows the spectral intensity profile I (ω) of the laser radiation source LS. The maximum intensity value I ₀ is at the center frequency ω ₀ . The frequency interval Δω = | ω ₂ −ω ₁ | in which the intensity decreases to half of the maximum value between the _two frequencies ω ₁ and ω ₂ is called a full width at half maximum (FWHM). It is clear that the laser lines contain different wavelengths that contribute to the total signal at different intensities. The spectral range within the full width at half maximum is also called the line core, and the outer region is called the line wing.

  FIG. 2B schematically shows a projection objective PO having two lens elements L1, L2 arranged coaxially with respect to the optical axis OA, which projection objective PO is placed in the object plane OS of the projection objective. An image forming system that forms an image of the pattern PAT in the region of the image plane IS optically conjugate with the object plane is shown. The aperture stop AS defines an effective image-side numerical aperture NA for imaging, and is disposed near the pupil plane of the objective system. The illumination light supplied from the illumination system is diffracted to the 0th diffraction order along the optical axis by the mask pattern PAT, and the -1st diffraction order at the outer edge of the beam path delimited by the aperture stop. It is schematically shown as a diffraction grating that diffracts to the + 1st order diffraction order.

  The broadband laser radiation source LS emits radiation having different wavelengths with different intensities, and according to the light color in the visible spectral range, the average wavelength is called “green” “g” and the longest wavelength is “red” “Red” ”is called“ r ”, and the shortest wavelength is called“ blue ”“ b ”. Accordingly, it goes without saying that a ratio in the wavelength from the deep ultraviolet range shorter than 260 nm must be applied. In the case of a system with insufficient color correction, the focal plane of the relatively shortest wavelength (b) is located close to the optical system, whereas the relatively long wavelengths (g) and (r) are Gradually focused further away. However, the substrate to be exposed, e.g. a semiconductor wafer, can only be guided into the focusing region for exactly one of the wavelengths, in this case the focal position of the other wavelengths is the surface to be exposed. It is located away from.

The focal position varies linearly with wavelength in a first order approximation (in the case of a color-corrected objective, this effect is only a quadratic function with respect to wavelength). Accordingly, the critical dimension change (ΔCD) rises in a quadratic function with respect to the wavelength change. From this it is clear that for the purposes of the present application, the chromatic aberration or its effect in the lithographic process is parameterized by the aberration parameter α, where:

  Along with the wavelength dependence of the focal position described here in detail, the chromatic aberration includes the wavelength dependence of the enlargement scale. This effect is also included in the aberration parameter α.

The second influential variable that affects the lithography process considered here is dynamic speckle. Speckle occurs as a result of uncontrolled or uncontrollable interference between different parts of the laser beam. Interference results in the loss of light at several locations, which corresponds to 100% contrast. The exact shape of the speckle pattern depends on the phase relationship of the different parts of the laser beam that change over time. In technical literature, these temporal intensity fluctuations are called “temporal speckles” or “dynamic speckles”. Taking into account that the only possibility to reduce contrast in these cases is the superposition of many different speckle patterns over the duration of the laser pulse, how fast the speckle pattern changes Question is brought. The number of independent speckle patterns results from dividing the duration of the laser pulse by the phase correlation time of the laser radiation. On the other hand, the correlation time comes from different frequencies of light in the laser radiation, i.e. it relates to the line shape of the emission intensity spectrum. For the purposes of this application, this correlation time or coherence time τ is derived from:

  The above equation shows that a relationship exists between the temporal coherence of the light source and its spectral bandwidth. Obviously, if both variables are defined effectively, they are inversely proportional to each other. Yet another more thorough analysis shows that the spectral shape of the radiation emitted by the light source also affects this relationship.

The aberration parameter α is proportional to the square of the bandwidth, whereas the coherence time τ is proportional to the reciprocal of the bandwidth. One important conclusion from this insight is that the product ατ ² is independent of bandwidth. Furthermore, this is based on the technical parameters of the bandwidth of the laser radiation source for the lithography process in order to obtain an optimization between the influence of the radiation source on chromatic aberration and the influence of the radiation source on the generation of dynamic speckles. This means that the spectral line shape that can be selected and parameterized with the line shape parameter ατ ² remains a free parameter.

  Next, the magnitude of the line shape parameters for various laser line shapes will be described in more detail below.

In many cases, technical books describe that the spectral shape of the laser line is a Lorentz profile.

However, this is only a rough approximation that is valid in the immediate vicinity of the center of the line. A more preferred technique widely used in laser physics is that of a modified Lorentz curve.

かつ、コヒーレンス時間に対しては次式が成り立つことは明らかである。

It is necessary that ν> 3 so that the chromatic aberration α does not diverge, that is, in order that optical lithography is completely possible. In this case, the following equation holds for chromatic aberration:

It is obvious that the following equation holds for the coherence time.

  The value for ν can be calculated from the FWHM (full width at half maximum) and the E95 value (95% of the energy is in this frequency range). In this case, these values are available through spectroscopic techniques.

From the measurement data for XLA-360 as a reference laser, it has been found that, for example, ν = 3.2. Plotting the line shape parameter ατ ² as a function of ν yields the curve shown in FIG. The point marks the commercially available “real” laser value ν = 3.2 corresponding to the value of the linear shape parameter ατ ² = 0.38. The limit of ατ ^{2 at} ν → ∞ is ατ ² = 1/12 (or ατ ² ≈0.083), and therefore only a slight improvement can be obtained beyond ν = 4.5.

Therefore, even when the assumed spectral line shape is limited to the modified Lorentz function, ατ ² can be improved more than three times with respect to the reference laser.

  If other spectral line shapes are possible, especially parabolic shapes, better improvements are possible further, as will be described below.

For different commercially available lasers, there are different indications for ν, precisely in the range of ν = 3.2 to ν = 3.75. It is assumed that these differences can only be partly attributed to the laser and can be attributed to measurement and evaluation with respect to the other parts. Thus, there remains a stage to evaluate how great the potential for individual laser improvement is in practice. The direct integration of the measured laser distribution for XLA-360 resulted in ατ ² ≈0.26, whereas the bypass by calculation fit for ν resulted in about ατ ² = 0.36 (FIG. 3). See).

  These numbers should be compared to the limit value 1 / 12≈0.083 for the modified Lorentz function and 9 / 125≈0.072 for the parabolic shape.

  To make the advantages of the invention somewhat clearer, a simple example will now be given with reference to FIG.

The line shape I (ω) shown in FIG. 4A is considered to be a (spectrum) flat top having a width Δ. In other words, this is, α = Δ ^2/12 from equation (1) is brought, the case where tau = 1 / delta is brought from the equation (4). Therefore, the product or line shape parameter is ατ ² = 1/12 (≈0.083).

Alternatively, assuming a Gaussian curve (as a function of frequency) having a width sigma in I (omega) (FIG. 4B), is α = σ ^2/2 from equation (1) brought, from equation (4) Is
τ = 1 / [√ (2π) σ]
Is brought about. Therefore, the product or line shape parameter ατ ² is ατ ² = 1 / (4Π) ≈0.080.

Thus, the product ατ ² is somewhat smaller in the Gaussian curve than the flat top shape. Thus, in the case of constant chromatic aberration, a somewhat shorter coherence length in the Gaussian curve, and hence less speckle, can be obtained. Alternatively, for the same speckle, chromatic aberration can be reduced if a Gaussian curve is used instead of a flat top. In other words, if it is important to reduce the effect of time-varying speckle on image generation compared to conventional systems and methods, and at the same time not to increase the effect of chromatic aberration on image generation, it is more gauss than flat-topped. A laser beam having a distributed shape is suitable for optical lithography.

The condition that the product ατ ^{2 according} to equations (1) and (4) is intended to be minimized leads to problems that are clearly defined mathematically. The optimized line shape of the following equation is determined by using Monte Carlo simulation. The laser line in this optimized deformation has the shape of a truncated parabola (see also FIG. 4C).

In this curve shape, α = Δ2 / 5 and τ = 3 / (5Δ), and the resulting product ατ ² is ατ ² = 9/125, corresponding to ατ ² ≈0.072. Accordingly, the products (line shape parameters) in different curve shapes are as follows.
Flat top: ατ ² = 1 / 12≈0.083
Gaussian curve: ατ ² = 1 / (4Π) ≒ 0.080
Parabolic shape: ατ ² = 9 / 125≈0.072

  Thus, the parabolic shape is actually better than the Gaussian or flat top distribution.

  These examples illustrate the importance of what happens in the region of the side edge of the intensity profile in the problem described here. For example, if 1% of the total energy in the side ridge is shifted, this shift has a greater order of influence than if 1% of the total energy in the line core is shifted. Thus, the laser line shape must have the smallest possible side edge contribution, where the term “side edge” substantially represents the transition region between the line core and the line wing. In this sense, a Gaussian curve has side edges that are wider than a parabola, and in the case of a Gaussian curve, the side edges theoretically go to infinity, whereas in the case of a parabola, it has only a finite extent. In contrast, when the side ridge is completely excised, as in the case of a flat top, this time, part of the core becomes a side ridge, which in the end is everything on the outside. It is because it is a side ridge. Therefore, in the case of a flat top shape, a region having very high strength becomes a side ridge. The parabola is therefore a very good compromise against the background with the underlying problem.

  With reference to FIG. 5, one possibility for influencing the spectral line shape in the case of an excimer laser is provided below by way of example. Excimer lasers are the primary light source used primarily in microlithography currently in the DUV and VUV range. Excimer lasers oscillate in many different spatial modes and have a low degree of spatial coherence compared to other types of lasers. In addition, a relatively broad spectrum is emitted, which means low temporal coherence. Yet another factor is a larger and higher emission power than with conventional non-laser light sources in the UV range. The natural bandwidth of an excimer laser can be in the range of a few hundred pm, and therefore should be greatly narrowed to provide a narrow bandwidth suitable for microlithography. Therefore, an excimer laser for microlithography is equipped with a so-called bandwidth narrowing module. In the case of the laser radiation source described in FIG. 5, the bandwidth narrowing module 510 forms the back terminator of the laser resonator and reduces the bandwidth of the emitted laser radiation by wavelength selective reflection. Have Adjacent to the rear extraction window of the amplification or gas discharge chamber 520 is a beam expansion unit 512 that includes four appropriately arranged prisms located behind in order. The expanded laser beam is incident on the echelle grating 530 in a Littrow configuration, so that the incident expanded laser beam is reflected onto itself. This basic structure is known per se. In this case, the spectral line profile of the laser is determined substantially by the spectral reflectance of the echelle grating 530. Grating blazed diffraction rules or any other technique can be used to define the grating height profile during manufacture to provide the desired spectral intensity distribution in the outgoing laser beam 550.

  In the exemplary embodiment, the height profile of the echelle grating 530 is a downward-opening parabolic scheme in which the laser radiation 550 emitted by the laser 500 can be substantially parameterized by the description shown in equation (10). It is designed to have the following spectral line shape.

  The desired spectral line shape of the emitted laser beam is pre-determined during manufacture of the reflective grating 530 for the bandwidth narrowing module. The height profile of the reflection grating that gives this line shape is then determined by back calculation. Next, the reflection grating is manufactured using a manufacturing method known per se, for example, according to the blaze diffraction law.

倍のスペックル低下を意味する。同じく、パルスストレッチャーによるパルス長の二倍化によって同じスペックル低下率を得ることができるが、この場合、伝達率が約２０％だけ低下するであろう。反対に、コヒーレンス時間を短縮する目的のための帯域幅の拡大は、対物系におけるより良好な色補正のための高価な手段を必要とすると考えられる。それとは対照的に、レーザのスペクトル線形状の提案する設定は、関連の悪影響を持たない技術的手段である。 We have shown how the lithography process can be improved by the proper setting of the spectral line shape. Four times the line shape parameter ατ ² means a coherence time that is twice as short for the same chromatic aberration, and thus

It means double speckle reduction. Similarly, the same speckle reduction rate can be obtained by doubling the pulse length with a pulse stretcher, but in this case the transmission rate will be reduced by about 20%. Conversely, increasing the bandwidth for the purpose of reducing coherence time is thought to require expensive means for better color correction in the objective system. In contrast, the proposed setting of the spectral line shape of the laser is a technical means that has no associated adverse effects.

100 Example of Microlithography Projection Exposure Apparatus 102 Primary Light Source 103 Optical axis 104 of Illumination System Beam Expander 190 Illumination System

Claims (16)

A projection exposure method for exposing a radiation-sensitive substrate arranged in an image plane area of the projection objective using at least one image of a mask pattern arranged in an object plane area of the projection objective,
Having a spectral intensity distribution I (ω) depending on the angular frequency ω,

An aberration parameter α according to

Generating laser radiation that can be further characterized by a coherence time τ according to
Introducing the laser radiation into an illumination system for generating illumination radiation directed onto a mask;
Imaging a mask pattern on a substrate using a projection objective;
Including
The spectral intensity distribution is set so that the condition ατ ² ≦ 0.3 holds for the line shape parameter ατ ² .
A method characterized by that. ^2. The projection exposure method according to claim 1, wherein the spectral intensity distribution is set so that a condition ατ ² ≦ 0.1 is satisfied with respect to the line shape parameter ατ ² . The spectral intensity distribution I (ω) corresponds to a Gaussian curve with a full width at half maximum σ,
α = σ ^2/2 and τ = 1 / (√2π σ)
Holds for the Gaussian curve,
The projection exposure method according to claim 1, wherein the projection exposure method is characterized.   The projection exposure method according to claim 1, wherein the spectral intensity distribution I (ω) has a parabolic shape.   The projection exposure method according to claim 1 or 2, wherein the maximum value of the spectral intensity distribution is in a wavelength shorter than 260 nm, particularly in an ultraviolet range of 193 nm. A projection exposure apparatus for exposing a radiation-sensitive substrate arranged in an image plane area of a projection objective using at least one image of a mask pattern arranged in an object plane area of the projection objective ,
A primary laser radiation source for emitting laser radiation;
An illumination system for receiving the laser radiation and generating illumination radiation directed on the mask;
A projection objective for generating an image of the mask pattern in the area of the image plane of the projection objective;
Including
The laser radiation source is designed to generate laser radiation having a spectral intensity distribution I (ω) that depends on the angular frequency ω;
The laser radiation is

An aberration parameter α according to

And a coherence time τ according to
The spectral intensity distribution is set so that the condition ατ ² ≦ 0.3 holds for the line shape parameter ατ ² .
A device characterized by that. The projection exposure apparatus according to claim 6, wherein the spectral intensity distribution is set so that a condition ατ ² ≦ 0.1 is satisfied with respect to the line shape parameter ατ ² . The spectral intensity distribution I (ω) corresponds to a Gaussian curve with a full width at half maximum σ,
α = σ ^2/2 and τ = 1 / (√2π σ)
Holds for the Gaussian curve,
The projection exposure apparatus according to claim 6 or 7, wherein   The projection exposure apparatus according to claim 6, wherein the spectral intensity distribution I (ω) has a parabolic shape.   10. The projection exposure apparatus according to claim 6, wherein the maximum value of the spectral intensity distribution is in a wavelength shorter than 260 nm, particularly in the deep ultraviolet range of 193 nm. A laser radiation source for generating laser radiation having a spectral intensity distribution I (ω) depending on an angular frequency ω for use in the projection exposure apparatus according to any one of claims 1 to 10. And
Laser radiation

An aberration parameter α according to

And a coherence time τ according to
The spectral intensity distribution is set so that the condition ατ ² ≦ 0.3 holds for the line shape parameter ατ ² .
A radiation source characterized by that.   12. A laser radiation source according to claim 11, characterized by the features of at least one characterization part of claims 7-10. A reflection grating for wavelength selective reflection of the laser radiation of the resonator of the laser radiation source such that the spectral intensity distribution of the laser radiation is substantially determined by the spectral reflectance of the reflection grating, 12. A bandwidth narrowing module, wherein a profile is formed to provide the spectral intensity distribution I (ω) having the line shape parameter ατ ^{2 for} which the condition ατ ² ≦ 0.3 holds. The laser radiation source according to claim 12. 14. A bandwidth narrowing module for a laser radiation source according to any one of claims 11 to 13, wherein the spectral intensity distribution of the laser radiation is substantially determined by the spectral reflectance of the reflecting grating. A reflective grating for wavelength selective reflection of the laser radiation of the resonator of the laser radiation source, such as
A height profile of the reflection grating is formed such that a spectral intensity distribution I (ω) depending on the angular frequency ω is provided for the laser radiation;
The laser radiation is

An aberration parameter α according to

And a coherence time τ according to
The condition ατ ² ≦ 0.3 holds for the line shape parameter ατ ² .
A module characterized by that. The height profile of the reflective grating is designed such that the spectral intensity distribution I (ω) substantially corresponds to a Gaussian curve with a full width at half maximum σ;
α = σ ^2/2 and τ = 1 / (√2π σ)
Holds for the Gaussian curve,
The bandwidth narrowing module according to claim 14.   15. The bandwidth narrowing module according to claim 14, wherein the height profile of the reflection grating is designed such that the spectral intensity distribution I (ω) has a substantially parabolic shape.

Images